Monday, July 5, 2010

What Is Radioactivity? What is Radiation?

Definition of Radioactivity and Radiation: Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability. The decomposition process is called radioactivity. The energy and particles which are released during the decomposition process are called radiation. When unstable nuclei decompose in nature, the process is referred to as natural radioactivity. When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivity.

Answer: There are three major types of natural radioactivity:

  1. Alpha Radiation

Alpha radiation consists of a stream of positively charged particles, called alpha particles, which have an atomic mass of 4 and a charge of +2 (a helium nucleus). When an alpha particle is ejected from a nucleus, the mass number of the nucleus decreases by four units and the atomic number decreases by two units. For example:

23892U -> 42He + 23490Th

The helium nucleus is the alpha particle.

  1. Beta Radiation

Beta radiation is a stream of electrons, called beta particles. When a beta particle is ejected, a neutron in the nucleus is converted to a proton, so the mass number of the nucleus is unchanged, but the atomic number increases by one unit. For example:

23490 -> 0-1e + 23491Pa

The electron is the beta particle.

  1. Gamma Radiation

Gamma rays are high-energy photons with a very short wavelength (0.0005 to 0.1 nm). The emission of gamma radiation results from an energy change within the atomic nucleus. Gamma emission changes neither the atomic number nor the atomic mass. Alpha and beta emission are often accompanied by gamma emission, as an excited nucleus drops to a lower and more stable energy state.

Alpha, beta, and gamma radiation also accompany induced radioactivity. Radioactive isotopes are prepared in the lab using bombardment reactions to convert a stable nucleus into one which is radioactive. Positron (particle with the same mass as an electron, but a charge of +1 instead of -1) emission isn't observed in natural radioactivity, but it is a common mode of decay in induced radioactivity. Bombardment reactions can be used to produce very heavy elements, including many which don't occur in nature.

Introduction to Molecular Geometry

Introduction to Molecular Geometry: Molecular Geometry or molecular structure is the three-dimensional arrangement of atoms within a molecule. It is important to be able to predict and understand the molecular structure of a molecule because many of the properties of a substance are determined by its geometry.

Molecular geometry angles contain a range of question, as we have large number of results about geometry angles. Molecular geometry angles exist as person as well as with lot of other geometric figures. Most of the molecular geometry angles figures are specified by their angles, like square, rectangles, triangles etc. since of so wide presence geometric angle have lot of results in them. These lots of results make a wide range of molecular geometry angles.

The Valence Shell, Bonding Pairs, and VSEPR Model

The outermost electrons of an atom are its valence electrons. The valence electrons are the electrons that are most often involved in forming bonds and making molecules.

Pairs of electrons are shared between atoms in a molecule and hold the atoms together. These pairs are called "bonding pairs".

One way to predict the way electrons within atoms will repel each other is to apply the VSEPR (valence-shell electron-pair repulsion) model. VSEPR can be used to determine a molecule's general geometry.

Predicting Molecular Geometry

Here is a chart that describes the usual geometry for molecules based on their bonding behavior. To use this key, first draw out the Lewis structure for a molecule. Count how many electron pairs are present, including both bonding pairs and lone pairs. Treat both double and triple bonds as if they were single electron pairs. A is used to represent the central atom. B indicates atoms surrounding A. E indicates the number of lone electron pairs. Bond angles are predicted in the following order:

lone pair versus lone pair repulsion > lone pair versus bonding pair repulsion > bonding pair versus bonding pair repulsion

Molecular Geometry